CppTools: Introduce LocatorData

Until now the locator filters CppLocatorFilter, CppClassesFilter and
CppFunctionsFilter used unnecessarily their own SearchSymbols instance.
The results were also saved separately, which was unfavorable since e.g.
the functions and classes data could be shared.

Starting with this patch an instance of LocatorData will serve all
mentioned filters.

This saves about 20MB of memory after indexing the Qt Creator project.

Change-Id: I8a34b67eb9fe0e5c68ba6e7c8f576389c78efc6f
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>